Many environmental pollution laws currently in effect in the United States and many foreign countries, require that exhaust emission reduction equipment in automotive vehicles be continuously monitored by on-board-diagnostic (OBD) systems. The function of the OBD system is to report system failure messages to the vehicle operator when emission control devices no longer meet the mandated emission levels. A key element of exhaust gas emission reduction systems is the catalytic converter, which in current automotive applications is used to simultaneously reduce the levels of carbon monoxide, oxides of nitrogen, and unburnt hydrocarbons in the exhaust.
Several OBD systems presently in use to monitor the performance of the catalytic converter employ a single-valued ratio of the number of voltage level transitions (switches) of two heated-exhaust-gas-oxygen (HEGO) sensors. To monitor the effectiveness of the catalyst, one HEGO is placed in the exhaust gas upstream from the catalytic converter, and the other HEGO is placed in the exhaust gas downstream of the catalytic converter. A controller receives output from the HEGO sensors and determines the number of voltage level transitions of the downstream HEGO relative to the number of voltage level transitions of the upstream HEGO.
The switching ratio of the upstream and downstream HEGO sensors can be used as a crude estimate of the oxygen storage capacity or activity of the catalyst (see, for example, "Detection of Catalyst Performance Loss Using On-Board Diagnostics," W. Clemmens, et al., Society of Automotive Engineers, Detroit Mich., 1990, paper 900062). This information is important because oxygen stored on the catalyst provides a source of oxygen for the oxidation of toxic gases in automotive engine exhaust. When the catalyst is no longer able to store sufficient oxygen, the performance of the catalytic converter declines and toxic gases can pass through the exhaust system and into the environment. When the estimated oxygen storage capacity falls below a predetermined level, the controller alerts the operator that exhaust system maintenance is necessary.
Although HEGO sensors are reliable and can function in the high-temperature, corrosive environment of an exhaust gas system, the switch ratio technique has poor resolution and only provides an ability to determine gross changes in the catalyst conversion efficiency. Additionally, the switch ratio technique requires recalibration when there are system changes in the engine or the catalyst.
The limit cycle characteristics employed by the switching ratio technique are affected by changes in control gains and system delays. These effects negatively impact the switch ratio monitors. For example, the frequency and amplitude of the limit cycle can vary depending upon the magnitude of control gains in the engine electronic control system. Variability in the frequency and amplitude of the limit cycle changes the characteristic wave forms from sensors positioned at the input to the catalyst. This distorts the results obtained from catalyst monitors based on comparison of switching frequencies. Additionally, other engine operating variables, such as air flow, temperature, rotational speed, and the like, effect the delay of the engine itself. This delay can alter limit cycle parameters, such as frequency and amplitude, and further reduce the measurement sensitivity and robustness of the switching ratio technique. Accordingly, an improved catalyst performance monitoring method is necessary to satisfy the more stringent exhaust emission controls mandated by current and future air pollution control laws.